Extended elements to the Standard may be defined by a data set producer or a user community. Extended elements are elements outside the Standard, but needed by the data set producer. If extended elements are created, they must follow the guidelines presented here. The Online_Linkage data element within Metadata_Extensions points to a formal document containing the necessary documentation for a subsequent metadata producer to reuse the extended element. If extended elements are used in a profile, the Profile_Name can also be included in the Metadata_Extensions compound data element. See Appendix E for Guidelines on Creating a Profile for the Content Standard for Digital Geospatial Metadata.

It should be noted here that metadata capture and exploitation tools need to be updated to adjust to these minor modifications. At the end of Section 7, Metadata Reference Information, the following elements were added in order to handle extended elements:

Extended elements may also be created for use with non-geospatial data holdings when this practice does not conflict with other Federal standards, directives, or statutes.

Metadata_Extensions=

0{Online_Linkage}n +

0{Profile_Name}1

If a metadata producer does not have the capability to provide online linkage, he or she may list the information in the data set.

The Online Linkage element references a structured file (or uniform resource locator) containing the following information about the extended element in the production rule dictated below: name, data type, domain values, short name, parent element, optionality (Mandatory, Mandatory-if-applicable, or Optional), cardinality, for example, (0, 1, ..., n) or ( > 1 but < 10), repeatability, definition, rationale, and source. The compound element Extension_Information is repeatable, but the primitive data elements occurring in Extension_Information are not. Note that these elements do not appear in the metadata record itself.

Extension_Information =

Name +

Short_Name +

Type +

[Domain | [0{Child}n | Rule]] +

1{Parent}n +

(Optionality) +

(Repeatability) +

(Definition) +

(Rationale) +

(Source)

The following rules must be followed when defining extended elements:

Extended elements must be formally documented within the hierarchical structure of the Standard (Section 0 - "Metadata"). When metadata extended elements are used, their existence must be documented in the Metadata Reference Information, element Metadata_Extensions. Every effort should be made to logically organize extended elements within the existing sections and structure. Where there is no logical or natural "fit", new section(s) and/or subsections may be defined.

Extended elements must not be used to change the name, definition, type, or domain of a standard element. In particular, an extended element cannot be nested under a data element.

Extended elements may be defined as compound and may include extended and standard elements as components. If a standard element is included in an extended compound element, no components of the standard element are changed.

Extended elements, like the standard element "Single_Date/Time" may appear in multiple places in the metadata set.

Extended_Element_Name -- (Mandatory) -- the name of the element. The name given to the element must not be the name of any other element in the Standard.

Domain: free text (Do not duplicate any other Standard element name.)

Definition -- (Mandatory) -- the definition of the element.

Domain: free text

Rationale -- (Optional) -- the reason for creating the Extended element, and its expected uses.

Domain: free text

Source -- (Mandatory) -- the name of the entity creating the Extended element.

Domain: free text

Type -- (Mandatory) -- the kind of value to be provided, or "compound" if the Extended element contains other elements.

Domain: integer, real, text, date, time, compound

Domain -- (Mandatory) -- valid values that can be assigned to the data element. The same rules as those for Standard elements are applied here. Domain: free text

Short Name -- (Mandatory) -- a unique short name consisting of eight alphabetic characters or less.

Domain: free text (Do not duplicate another short name used by the Standard or any other short name in the Extension Registry.)

Parent -- (Mandatory) -- The name of the element(s) under which this element may appear. The name(s) may be standard or other extended element(s).

Domain: none, free text (Must be the name of an existing standard or extended element. If the extended element is not part of any other compound element, its parent is the section name as defined by the Standard. If the extended element is defining a whole new section, then its parent is Section 0 - "Metadata".)

Child -- (Mandatory-if-applicable) -- The name of the element(s) which may appear under this element. The name(s) may be standard or other extended element(s).

Domain: none, free text (Must be the name of an existing Standard or extended element.)

Rule: production rule for the element, specified using the form given in this Standard.